Electrical energy generation in the United States relies on a variety of energy sources such as fossil, nuclear, solar, wind and hydroelectric. With the concern of the dwindling supply of fossil fuel, one of the great challenges of energy supply chains is balancing supply with demand. In particular, managing the intrinsic intermittency of renewable sources of energy such as wind or solar is key to enable their adoption at large scale. Part of the problem is the inability to store electrical energy in an efficient and cost effective way. Electrochemical cells using liquid metals in alloying/dealloying reactions have been developed but generally operate at low voltages of about 1 volt or less. Enabling higher voltage cells while retaining the use of low cost materials would significantly decrease the cost of these devices and further improve their efficiency.
Ion selective membranes have been used as separators in electrochemical cell systems. For example, in a traditional zebra (Na/NiCl2) battery, an ion selective Na+ conductive β″-alumina ceramic membrane may be inserted between the electrodes to prevent the reaction of Na with the electrolyte as well as the irreversible back reaction of Ni2+ upon direct contact with the negative Na electrode. During charging, the solid Ni is oxidized to Ni2+ ion at the positive electrode while Na+ is reduced to liquid Na at the negative electrode. Ideally, an ion selective membrane should be as thin as possible so that its electrical resistance is as low as possible in order to allow maximum current to flow. However, a thin membrane is difficult to manufacture without pinholes, and a thin membrane lacks structural integrity and is subject to mechanical failure. Thus, these types of membranes have operational and manufacturing issues. For example, in the zebra (Na/NiCl2) battery, the drawbacks of the β″-alumina ceramic membrane are (1) the membrane requires a complex manufacturing process that includes many steps, high temperature sintering and a controlled environment to achieve the intricacy of the β″ crystal structure; (2) the membrane is mechanically vulnerable and limits the lifetime of the battery, e.g., by failure under repeated thawing; (3) the thin ceramic membrane is fragile and increasingly vulnerable at larger scale; (4) the membrane requires minimal operating temperature, e.g., >200° C., to be practical because of its limited conductivity; and (5) the membrane is limited to Na+ itinerant ions.